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  1. #1
    Join Date
    Sep 2007
    Location
    Hinesville,GA
    Posts
    44

    system design pressure

    whats going on gentlemrn i ave a ? i am trying to desgn a system and i am wondering what is the correct way to find the right design pressure for the system when you are making the duct system i have an 8 ton H/P system which makes it about 3200 cfm's of air that needs to be moved, i have every manul you can think of but the books just are not getting the point across to me, i am in iraq contracting and i figure we need some good duct systems around here because i am tired of these little splits can anyone help

  2. #2
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,705
    If your doing a design and build, you can choose what you need your static to be.

    You just need to order your commecial equipment with the proper blower and drive.
    When space is limited in commercial, 1" and higher static is not unusal.

    So the question is, what static is available from the equipment you have selected?
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  3. #3
    Join Date
    Jan 2008
    Location
    lexington, kentucky
    Posts
    50
    been there has a good point. Also, once yo find the esp, use a good duct calculator for sizing the duct.

  4. #4
    Join Date
    Apr 2007
    Location
    Dallas TX
    Posts
    192
    a very brief class:
    a load calculation will indicate how many cfm is needed at the supply ducts for each room.
    cfm + length/friction and a ductulator will give you a duct size.
    a 'rule-of-thumb' for return size ... 1 sq. ft. per ton.

  5. #5
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,705
    Hope your refering to a return grille.

    Other wise, at 1 sq ft per ton, he'd have a 20 x 36 return duct for a 5 ton unit.
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  6. #6
    Join Date
    Apr 2007
    Location
    Dallas TX
    Posts
    192
    of course that infers that it is not in a return air chase that is only one 2x4 wide.

  7. #7
    Join Date
    Dec 2007
    Location
    oregon
    Posts
    885
    A duct calculator uses 100 feet as the guide line, so be careful when using your "ductulator". You should also determine the total duct friction loss with fittings, registers, grilles,coil and filters. The formula for friction loss is:

    FRICTION LOSS PER 100 FEET= SYSTEM DESIGN PRESSUREX100/ TOTAL EQVUIVELANT LENGTH. this will help you as you compare the friction of your duct system with the availiable static of your unit.
    Find your manual d book and look up the fittings in the back of the manual. it has the TEL(total equiv. length) of most fittings. the ASP (available static pressure) should be in the manuals with your package units.
    Genius = The guy who can do anything...except make a living!

  8. #8
    Join Date
    Sep 2007
    Location
    Hinesville,GA
    Posts
    44

    system design pressure

    The building needs a total of 6400 cfms i have split it into two halves each will be conditioned by 1 8 ton heat pump system, i am just a little fogged about how to pick a good pressure for the system or i how i go about doing it.

  9. #9
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,705
    You need the fan data/specs for the equipment your putting it, to know how much static you have to work with.
    Plus the duct layout/length, and CFM for the branches, Drop of grilles and regs.
    After subtracting all of that from your ASP, you will be able to determine what FR to use.
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  10. #10
    Join Date
    Sep 2007
    Location
    Hinesville,GA
    Posts
    44

    system design pressure

    I have an ACCA approved duculator, i have yet to recieve the package units because they are in route from dubai. do you add all of the resistence in the duct system or just from the filter to the register of the longest run.I have manual D,N,J just without having a working example it is a little hard, it has been a while sice i have had to do this. I appreciate the help guys, just trying to make sure i am going to put out a quality job.

  11. #11
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,705
    Yes.
    All pressure drops count.

    If I need the specs on a unit before I get it.
    I either call my supplier and have them e mail it to me, or I download it from the manufacturers website.

    On D&B(design and build)
    I usually have a print, I layout my duct work on, figure my static, and then order the unit with a blower and drive package for my duct design.
    Its sometimes easier then trying tomake a standard blower and drive work, when you don't have a lot of room for your duct.
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  12. #12
    Join Date
    Sep 2007
    Location
    Hinesville,GA
    Posts
    44

    system design pressure

    So you find the available static pressure you have from the blower add all of your resistence for your coil registers any thing that would cause you a pressure drop and subtract it from your ASP, and that will leave you with your system design pressure.

  13. #13
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,705
    Yes.

    On some commercials, your main trunk is so long, or has that many ells and transitions in it that with branch lengths regs, and grilles, you can't use a standard blower/drive package.


    I'm working on one now, that the engineer has speced out for blower drive packages for 1" static. Not enough room in the ceiling for a standard package.
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